Conventionally, a Light Emitting Diode (LED) has been widely employed as an illumination light source. A LED has a feature that a response speed is faster than those of an incandescent bulb or a fluorescent lamp. A visible light communication technique is under study which exploits this feature to blink a LED at a rate imperceptible to the human eyes to superimpose information on illumination light emitted by the LED and perform communication (see, e.g., non-patent literature 1).
Visible light communication is considered to be used for, e.g., the purposes of communication at locations where the use of radio waves is restricted, information delivery limited to the range in which light propagates, such as an indoor space, or an Intelligent Transport System (ITS).
For example, an information transmission system disclosed in patent literature 1 includes a light-emitting means for illuminating a predetermined region to transmit information, and a light-receiving means for decoding the information from images obtained by capturing the predetermined region on a time-series basis. In the information transmission system, the predetermined region emits light modulated in color by multilevel-coded modulation to at least three levels in accordance with the information, and the light-receiving means decodes the information on the basis of multilevel-coded color modulation information of the predetermined region.
An optical module has further been proposed which includes at least two primary light sources that emit primary color light and modulates the color coordinates of the light emitted by the primary light sources in accordance with data to be embedded, to incorporate the data in the light (see, e.g., patent literature 2). Since the sensitivity of the human eye to changes in color is lower than changes in intensity, the use of the optical module for an illumination system allows embedding of data in emitted light without degrading the function of illumination.